What Causes Vision Loss in Glaucoma?

What is it that blinds in glaucoma? At first blush, this seems like a straightforward question. But here’s a hint: what blinds in glaucoma is not elevated pressure in the eye.

True, high ocular pressure is an important risk factor, the only one that is treatable. It is also true that lowering pressure can slow vision loss from glaucoma. Unfortunately, though, vision loss can continue even when ocular pressure is kept in check, and lowering pressure cannot restore vision that has already been lost. To understand why requires us to examine the anatomy of the eye in a bit more detail.

The retina of the eye is a transparent sheet of nerve cells (called neurons) that transforms light signals into electrical impulses that travel along the optic nerve to the brain. The optic nerve is comprised of about 1.5 million thin fibers that originate from specialized neurons in the retina called ganglion cells. The ganglion cell fibers leave the eye at the optic disc, where many of the characteristics of glaucoma make themselves known during an eye exam.

Though we do not know why, the retinal ganglion cells and their fibers in the optic nerve are especially vulnerable to injury due to pressure. This vulnerability increases as we age; in fact, age is the greatest risk factor for glaucoma. As the population ages, there is an increase in the number of cases that involve injury to these neuronal structures without high ocular pressure and where the injury continues even when the pressure is lowered by drugs or surgery.

Thus, what blinds in glaucoma is damage to the retinal ganglion neurons and their fibers in the optic nerve. Because these structures are part of the central nervous system, just like the brain and spinal cord, their ability to heal is extremely limited. Because of this, many researchers think of glaucoma as an age-related disease of the brain, like Alzheimer’s or Parkinson’s disease.

Researchers hope that by studying other neurodegenerative diseases like these, we can identify common causes that will help us develop new treatments that directly target those aspects of glaucoma that are truly blinding.

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Article by David J. Calkins, PhD, Director of Research at the Vanderbilt Eye Institute, the Vanderbilt Brain Institute, and the Vanderbilt Vision Research Center, Vanderbilt University Medical Center, in Nashville, TN.